0325 : Catabolism of leucine in the heart inhibits glucose transport

Abstract

Branched-chain amino acids like leucine induce insulin resistance in muscle and adipose tissues. The mechanism explaining leucine action involves mTOR/p70S6K signaling. This pathway is activated by leucine and is implicated in the stimulation of an insulin negative feedback loop. Knowing that insulin-resistance participates in diabetic cardiomyopathy, we were interested in studying leucine action in cardiomyocytes. Primary cultured adult rat cardiomyocytes were pretreated with different concentrations of leucine (1 to 10mM) during different periods of time (up to 20h) before being exposed to insulin (3x10-9M, 30min). Insulin increased glucose transport. This correlated with the increase of PKB and AS160 phosphorylation, both known to regulate GLUT4 translocation to the plasma membrane allowing glucose uptake. 1h pre-incubation with leucine stimulated mTOR/p70S6K pathway. This is accompanied by a decrease in PKB and AS160 phosphorylation but, surprisingly, insulin-stimulated glucose uptake was preserved. On the other hand, a longer incubation (14h) with leucine induced a drastic decrease in glucose transport. The mTOR/p70S6K inhibitor rapamycin did not prevent this inhibition. The non-metabolized leucine analog BCH had no effect on the insulin-induced glucose uptake. By contrast, intermediates of leucine catabolism, alpha-ketoisocaproate and ketone bodies inhibited glucose uptake similarly to leucine. This inhibition is clearly independent of insulin signaling because leucine also inhibited basal glucose transport and glucose uptake stimulated by the insulin-unrelated pathway involving AMPK. The leucine-mediated inhibition of glucose transport resulted from the inhibition of GLUT4 translocation. The exact molecular mechanism downstream leucine’s metabolites and responsible for the inhibition of GLUT4 translocation and glucose uptake is under investigation. Leucine catabolism reduces cardiac glucose transport independently of insulin signaling by an undefined mechanism.